Damascus: A suicide bomber on Wednesday struck at the heart of Syria's top command, killing three of President Bashar al-Assad's top security figures in an attack claimed by rebels who warned of more carnage to come.

The bombing, which an official blamed on a bodyguard attending a meeting of security chiefs at their Damascus headquarters, prompted US Defence Secretary Leon Panetta to warn the situation in Syria was "spinning out of control."

The attack which targeted Assad's inner core for the first time in a 16-month uprising came ahead of a showdown between the West and Russia and China over a UN resolution calling for sanctions that now appears to have been pushed back by a day to Thursday.

A Syrian security official told AFP the bombing was carried out by a bodyguard of one of the ministers or security chiefs at the meeting. The attacker had been wearing an explosives belt.

The blast killed defence minister General Daoud Rajha, Assad's brother-in-law Assef Shawkat and Hassan Turkmani, the head of the regime's crisis cell, head of National Security, the channel and security officials said.

Among those wounded was interior minister Mohammed al-Shaar and General Hisham Ikhtiyar.

The rebel Free Syrian Army claimed responsibility for the attack which came as its fighters battled forces loyal to Assad across Damascus for a fourth straight day.

The FSA command "announces the good news of the outstanding operation this morning that targeted the National Security headquarters and the killing" of the officials "responsible for barbaric massacres," it said.

The rebels said the attack, part of Operation Damascus Volcano launched on Monday, "is the first in a series... aimed at bringing down Assad and the pillars and symbols of the regime, whether civilian or military."